1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for removing and/or replacing a section of a subsea control module.
2. Discussion of the Background
Subsea oil and gas exploration becomes more challenging as the exploration depth increases. Complex devices are disposed on the ocean floor for extracting the oil and for the safety of the oil equipment and the environment. These devices have to withstand, among other things, high pressures (from 3,000 to 60,000 psi (200 to 4000 bar) or more) and highly corrosive conditions. Although precautions are taken when building these devices, component parts of these devices wear out with time and need to be replaced.
As these parts are disposed on the ocean floor (sometimes more than 2000 m below sea level) and sometimes are provided inside larger components, access to them may be problematic. For example, FIG. 1 illustrates a lower blowout preventer stack (“lower BOP stack”) 10 that may be rigidly attached to a wellhead 12 upon the sea floor 14, while a Lower Marine Riser Package (“LMRP”) 16 is retrievably disposed upon a distal end of a marine riser 18, extending from a drill ship 20 or any other type of surface drilling platform or vessel. As such, the LMRP 16 may include a stinger 22 at its distal end configured to engage a receptacle 24 located on a proximal end of the lower BOP stack 10.
In typical configurations, the lower BOP stack 10 may be rigidly affixed atop the subsea wellhead 12 and may include (among other devices) a plurality of ram-type blowout preventers 26 useful in controlling the well as it is drilled and completed. The flexible riser provides a conduit through which drilling tools and fluids may be deployed to and retrieved from the subsea wellbore. Ordinarily, the LMRP 16 may include (among other things) one or more ram-type blowout preventers 28 at its distal end, an annular blowout preventer 30 at its upper end, and a MUX pod (in reality two, which are referred to in the industry as blue and yellow pods) 32.
When desired, the ram-type blowout preventers of the LMRP 16 and the lower BOP stack 10 may be closed and the LMRP 16 may be detached from the lower BOP stack 10 and retrieved to the surface, leaving the lower BOP stack 10 atop the wellhead. Thus, for example, it may be necessary to retrieve the LMRP 16 from the wellhead stack in times of inclement weather or when work on a particular wellhead is to be temporarily stopped.
Also, when a part of the LMRP 16 fails, the entire LMRP 16 may need to be raised on the ship 20 for repairs and/or maintenance. One such part that may require maintenance from time to time is the MUX pod 32. A conventional MUX pod system 40, is shown in FIG. 2 and may provide between 50 and 100 different functions to the lower BOP stack and/or the LMRP and these functions may be initiated and/or controlled from or via the LMRP.
The MUX pod 40 is fixedly attached to a frame (not shown) of the LMRP and may include hydraulically activated valves 50 (called in the art sub plate mounted (SPM) valves) and solenoid valves 52 that are fluidly connected to the hydraulically activated valves 50. The solenoid valves 52 are provided in an electronic section 54 and are designed to be actuated by sending an electrical signal from an electronic control board (not shown). Each solenoid valve 52 is configured to activate a corresponding hydraulically activated valve 50. The MUX pod 40 may include pressure sensors 56 also mounted in the electronic section 54. The hydraulically activated valves 50 are provided in a hydraulic section 58 and are fixedly attached to the MUX pod 40 (i.e., a ROV vehicle cannot remove them when the same is disposed on the seafloor).
In typical subsea blowout preventer installations, multiplex (“MUX”) cables (electrical) and/or lines (hydraulic) transport control signals (via the MUX pod and the pod wedge) to the LMRP 16 and lower BOP stack 10 devices so specified tasks may be controlled from the surface. Once the control signals are received, subsea control valves are activated and (in most cases) high-pressure hydraulic lines are directed to perform the specified tasks. Thus, a multiplexed electrical or hydraulic signal may operate a plurality of “low-pressure” valves to actuate larger valves to communicate the high-pressure hydraulic lines with the various operating devices of the wellhead stack.
A bridge between the LMRP 16 and the lower BOP stack 10 is formed that matches the multiple functions from the LMRP 16 to the lower BOP stack 10, e.g., fluidly connects the SMP valves 50 from the MUX pod provided on the LMRP to dedicated components on the BOP stack or the LMRP. The MUX pod system is used in addition to choke and kill line connections (not shown) or lines that ensure pressure supply to, for example, the shearing function of the BOPs.
The bridge is shown in FIG. 3 and may include a pod wedge 42 configured to engage a receiver 44 on the BOP stack. The pod wedge 42 has plural holes (not shown), depending on the number of functions provided, that provide various hydraulic and/or electrical signals from the LMRP 16 to the lower BOP stack 10. However, it is noted that the pod wedge 42 is designed with a given number of functions (holes) and after being deployed, the MUX pod system cannot be modified to handle more functions.
Examples of communication lines bridged between LMRPs and lower BOP stacks through feed-thru components include, but are not limited to, hydraulic choke lines, hydraulic kill lines, hydraulic multiplex control lines, electrical multiplex control lines, electrical power lines, hydraulic power lines, mechanical power lines, mechanical control lines, electrical control lines, and sensor lines. In certain embodiments, subsea wellhead stack feed-thru components include at least one MUX pod connection whereby a plurality of hydraulic control signals are grouped together and transmitted between the LMRP 16 and the lower BOP stack 10 in a single mono-block feed-thru component as shown, for example, in FIG. 3.
In conventional MUX pods, when one or more of the solenoid valves 52 or any of the various other instruments and components require service or replacement, which happens from time to time, the whole MUX pod 40 has to be brought to the surface. However, as the MUX pod 40 is bolted to the LMRP, it is necessary that the entire LMRP be brought to the surface for repair. This operation is disrupting for the functioning of the well as the drilling or oil extraction has to be stopped, which involves production losses. In addition, the size and weight of the MUX pod 40 and the LMRP are large (sometimes in the range of tens to hundreds of tons), which makes the entire retrieval process not only time consuming but dangerous.
An approach to limit the disruption of oil extraction has been presented in U.S. Pat. No. 7,216,714 to G. Reynolds, the entire disclosure of which is incorporated here by reference. U.S. Pat. No. 7,216,714 uses a control module 60 (shown in FIG. 4, which corresponds to FIG. 5 of U.S. Pat. No. 7,216,714) that combines a pilot valve (solenoid valve) 62 with a hydraulically activated valve 64, both disposed in a single casing 66. The control module 60 has a connector 68 that connects to a receiver 70 that is fixedly attached to the BOP stack. Thus, when the pilot valve 62 fails, the entire control module 60 may be detached from receiver 70 and brought to the surface for repair by use of a Remotely Operated Vehicle (ROV). In this way, the BOP stack remains on the well head. This process minimizes the down time of the oil rig.
However, this process is still cumbersome as both the hydraulically activated valve and the solenoid valve need to be removed and brought to the surface. Once there, the control module 60 has to be disassembled and only the failed part replaced with a new part. However, the weight and size of the control module may be significant, thus imposing considerable power requirements on the ROV vehicle. Another disadvantage of the existing devices is that most of the time there is no need to bring to the surface the SPM valves as these valves are more reliable than the electro-hydraulic valves. Accordingly, it would be desirable to provide systems and methods that are faster and simpler than the afore-described approaches.